1. TECHNICAL FIELD
This invention relates to an improved resin-containing coating solution which, when applied to a fabric substrate and processed to coagulate the resin, results in an improved waterproof, microporous, moisture-vapor-permeable, flame-resistant fabric. The coated fabric retains good moisture permeability with durable flame resistance that remains characteristic of the fabric even following multiple launderings. Procedures for making such fabrics are also described.
2. BACKGROUND
Coated fabrics suitable for use as activewear, rainwear and tentage function by blocking the pores of a woven, knitted or non-woven fabric with a cohesive polymer film which acts as a physical barrier against wind, water, and in the case of protective workwear, aggressive chemicals, oils, and greases. This barrier or coating distinguishes polymer coatings from chemical finishes which merely coat the individual fibers of a fabric without blocking the pores, and repel fluids by surface tension effects. Microporous coated fabrics repel water from the outside yet allow perspiration and moisture vapor to escape from the inside. Moisture is transmitted through a tortuous physical pathway produced in the cellular film or coating resulting from the art-recognized wet coagulation process, as described in more detail below. Polymeric coatings have initially been based upon rubber or synthetic or fluorocarbon rubbers, and more recently, polyurethanes, acrylics, silicone elastomers and polyvinylchlorides.
Fashion and leisurewear, particularly rainwear, require that the coated material be attractive with good drape and handle, that it be water repellent, although not necessarily for prolonged use in heavy rain, and that the fabric retain these properties after drycleaning or laundering.
There are several fabrics available that satisfy the conflicting requirements of waterproofness and breathability. One example is Entrant, which is a woven nylon fabric coated with a microporous polyurethane film formed by the so-called wet coagulation technique as described in U.S. Patent 4,429,000 to Toray Industries, Inc. Other polyurethane coated fabrics are described in U.S. Patent 3,360,394 to Griffin. In the wet coagulation method, a thin, microporous polyurethane layer is formed on a base fabric by applying a coating solution of a polyurethane dissolved in a polar organic solvent that will solubilize the polyurethane yet is miscible with water. The polymer solution is applied to the fabric substrate by knife coating or the like, then immersed in a bath of water which selectively dissolves or mixes with the organic solvent, exchanges water for the polar solvent and causes the previously dissolved polyurethane to coagulate, leaving a thin, microporous coating having a cellular substrate on the fabric. Surface pores that result are generally one micron or less in diameter. Such pores are small enough to exclude water droplets and yet they provide a tortuous physical pathway from the base fabric to the coating surface, to allow water vapor to pass through the fabric. The coating is a thin polymeric (polyurethane) film.
Typical coating solutions contain a resin, usually a polyurethane elastomer, optionally a water repellent agent, a thickener, a surfactant and possibly other adjuvants, all dissolved in a water-miscible polar organic solvent such as dimethylformamide, N-methyl-2-pyrrolidone, dimethylacetamide or dimethylsulfoxide. The coagulating bath contains water with up to 20% by weight of the same or a compatible polar solvent. Coating viscosity must be carefully controlled to adjust penetration and interstitial strike-through, especially on loosely-woven and textured fabrics.
In addition to apparel uses, microporous coated fabrics are used for tentage and tarpaulins. Both uses demand waterproofness as a primary requirement. For both a high water vapor transmission rate is very desirable, in order to prevent the accumulation of condensed moisture on the inner surface of the structure. Flame resistance, particularly in tents for both military and civilian use, is extremely important for articles constructed of microporous coated fabrics. As with the other components mentioned above, the flame-resistant coating must be durable to cleaning, usually laundering in water. Flame-resistant microporous coated fabrics are included within this invention.
Microporous coatings, as described above and elsewhere, are made by dissolving a polymer in a water-miscible solvent, then mechanically applying this solvent solution as a coating to a fabric. The thus-coated fabric is then immersed in a non-solvent, such as water. The microporous structure of the coating is completed when all the solvent has been displaced with the non-solvent, leaving a microporous layer on the base fabric.